This invention relates to a circuit for use with a sensor responsive to magnetic fields, and has particular, though not exclusive application, to what is known as a torque sensor.
Magnetoelastic torque sensors are known in the art. They are used to sense a magnetic field emanated by a magnetoelastic element mounted to a shaft due to the torque resulting from rotation or attempted rotation of a shaft. The rotation of any shaft from one end to drive a load at the other end generates torque in the shaft due to differential angular displacement (which may be very small) between the point at which drive is applied and the point at which the load is driven. If magnetoelastic means generating a magnetic field is associated with a shaft the field becomes distorted by the torque in the shaft and such distortion can be sensed as a measure of torque.
It has further been proposed to sense the distortion of the magnetic field away from its quiescent (non-torque) state by arrangements of one or more coils placed in the close vicinity of the shaft in a non-contacting arrangement allowing the shaft to rotate within the coil assembly. More particularly, because of the small changes in magnetic field due to applied torque, it has been proposed to use saturable coils (inductors) driven into saturation so that the point of saturation of a coil, with respect to a drive current, depends on the magnetic field associated with the shaft. The points of saturation for the opposite drive polarities in the coil become unbalanced due to the presence of an external field and it is this unbalance that is sensed. Circuitry responsive to the point of saturation is used to develop an output signal representing torque.
Magnetoelastic torque sensing in general can thus be divided into two parts: one is the magnetoelastic element and the arrangement for mounting one or more sensors with respect to the shaftxe2x80x94the torque transducer; and the other is the sensing circuit connected to the one or more sensors and processing the resultant signalsxe2x80x94the signal conditioner. The sensor of interest here is one or more coils.
The present invention is concerned with the sensing circuit. The actual arrangement of the one or more magnetic coils is not the concern of this invention.
By way of example, a magnetometer using a saturating inductor and the associated circuitry is described in U.S. Pat. No. 5,124,648 (Webb and Brokaw). More detailed information on the mounting of torque sensor coils (inductors) with respect to rotary shafts is to be found in U.S. Pat. No. 5,520,054 (Garshelis). Reference may also be made to a technical paper published by the Society of Automotive Engineers (SAE), xe2x80x9cDevelopment of a Non-Contact Torque Transducer for Electric Power Steering Systemsxe2x80x9d, SAE Technical Paper Series, No. 920707, I. J. Garshelis, K. Whitney and L. May, reprinted from: Sensors and Actuators, 1992 (SP-903), International Congress and Exposition, Detroit, Mich., Feb. 24-28, 1992, pp. 173-182. Further discussion of torque transducers is found in xe2x80x9cA Single Transducer for Non-Contact Measurement of the Power, Torque and Speed of a Rotating Shaftxe2x80x9d, I. J. Garshelis, C. R. Conto and W. S. Fiegel, SAE Technical Paper Series, No. 950536, reprinted from: Sensors and Actuators (SP-1066), International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995, pp. 57-65, particularly pp. 58-59.
An example of a commercially-offered magnetoelastic torque transducer is that offered under the trade mark TorqStar by the Lebow Products division of Eaton Corporation, 1728 Maplelawn Road, Troy, Mich., U.S.A.
The signal conditioner circuit described in U.S. Pat. No. 5,124,684 is used with a single winding magnetometer sensor xe2x80x94 a saturable inductor. The inductor is grounded and the ungrounded end driven by an oscillator. An integrator arrangement makes a difference measurement at the ungrounded end due to an unbalanced sensed field and actively nulls the field in the inductor core. A voltage indicative of the unbalancing magnetic field is measured by an amplifier and used to create a feedback current into the inductor derived from the output voltage to null the imbalance. This nulling current is applied through a resistor to act as a current source but the value of the current is dependent also on the inductor resistance which is uncertain and temperature sensitive. U.S. Pat. No. 5,039,945 addresses this problem and proposes a solution having a great deal of added complication.
Another circuit for measuring a magnetic field is described in Patent Abstracts of Japan vol. 009, No. 186 (p. 377) and JP 60057277A. The Abstract discloses an amplifier connected in a self-excited oscillator including a feed-back transformer having a saturable core sensitive to an external magnetic field. The transformer has a first winding connected at the output of the amplifier and a second winding connected to the input of the amplifier. An adjustable resistor in series with the first winding is adjusted to give the best possible square wave output from the amplifier. The voltage output which is obtained across the series connection of the first winding and the adjustable resistor is applied to a measuring amplifier.
The present invention is based on the recognition that the unbalance current in the sensor coil due to the external field can be used directly as a measurement parameter, thereby avoiding the need to generate the nulling or compensating current of the prior art.
Aspects and features of the present invention for which protection is presently sought are set out in the accompanying claims.